Traditional techniques for introducing a printed image onto a recording material include letterpress printing, gravure printing, and offset lithography. All of these printing methods require a plate. To transfer ink in the pattern of the image, the plate is usually loaded onto a plate cylinder of a rotary press for efficiency. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing cylinders, in contrast, contain a series of wells or indentations that accept ink for deposit onto the recording medium. Excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
The term "lithographic," as used herein, is meant to include various terms used synonymously, such as offset, offset lithographic, planographic, and others. By the term "wet lithographic," as used herein, is meant the type of lithographic printing plate where the printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced. In a dry lithographic printing system that does not utilize water, the plate is simply inked and the image transferred directly onto a recording material or transferred onto a blanket and then to the recording material.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typically subject to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the image to the plate and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining and anodizing affect both the performance and the durability of the printing plate. Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are also well known in the art, and need not be further described herein. The aluminum support is thus characterized by having a porous, wear-resistant hydrophilic surface which specifically adapts it for use in lithographic printing, particularly where long press runs are required.
The plates for an offset press are usually produced photographically. The aluminum substrate described above is typically coated with a wide variety of radiation-sensitive materials suitable for forming images for use in the lithographic printing process. Any radiation-sensitive layer is suitable which, after exposure and any necessary developing and/or fixing, provides an image which can be used for printing. Lithographic printing plates of this type are usually developed with an aqueous alkaline developing solution which often additionally comprises a substantial quantity of an organic solvent.
To prepare a wet plate using a typical negative-working substractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), thereby exposing the hydrophilic surface of the aluminum plate.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As is evident from the above description, photographic platemaking processes tend to be time consuming and require facilities and equipment adequate to support the necessary chemistry. Efforts have been made for many years to manufacture a printing plate which does not require development or which only uses water for development. In addition, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic radiation, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced closely to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image (see, e.g., U.S. Pat. No. 4,911,075). Because of the ready availability of laser equipment and its amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. These systems include:
1) Argon-ion, frequency-doubled Nd-YAG and infrared lasers used to expose photosensitive blanks for traditional chemical processing, as for example described in U.S. Pat. Nos. 3,506,779; 4,020,762; 4,868,092; 5,153,236; 5,372,915; and 5,629,354. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation, with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate, as for example described in U.S. Pat. No. 4,132,168.
However, the need for high writing speeds, coupled with the constraint of the low-powered lasers favored by industry, has resulted in a requirement for printing plates that have a very high photosensitivity. Unfortunately, high photosensitivity almost always reduces the shelf life of these plates.
2) Another approach to laser imaging uses thermal-transfer materials, as for example described in U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946; and 5,395,729. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. The transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent polymer sheet with the unirradiated transfer material still on it leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic, and the acceptor material is hydrophilic.
Plates produced with transfer type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred. Airborne dirt can create an image quality problem depending on the particular construction. In addition, because the transfer process involves melting and resolidification of material, image quality further tends to be visibly poorer than that obtainable with other methods.
3) Other patents describe lithographic printing plates comprising a support and a hydrophilic imaging layer which, upon imagewise laser exposure, becomes oleophilic in the exposed areas while remaining hydrophilic in the unexposed areas, as for example disclosed in U.S. Pat. Nos. 3,793,033; 4,034,183; 4,081,572; and 4,693,958. However, these types of lithographic printing plates suffer from the lack of a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas, with the result that image quality on printing is poor.
4) Early examples utilizing lasers used the laser to etch away material from a plate blank to form an intaglio or letterpress pattern, as for example described in U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal an oleophilic underlayer, as for example described in U.S. Pat. No. 4,054,094. These early systems generally required high-power lasers, which are expensive and slow.
More recently, other infrared laser ablation based systems for imaging hydrophilic plates have been developed. These operate by laser-mediated removal of organic hydrophilic polymers which are coated onto an oleophilic substrate such as a polyester/metal laminate or onto an oleophilic polymer coating on a metal support. Use of these materials between the ablation coating and the heat absorbing metal support provides a thermal barrier material which reduces the amount of laser energy required to ablate or physically transform the hydrophilic surface layer, as for example described in U.S. Pat. Nos. 5,353,705; and 5,570,636. Laser output either ablates one or more plate layers, or physically transforms, the oleophobic or hydrophilic surface layer, in either case resulting in an imagewise pattern of features on the plate.
One problem with this approach is that the hydrophilic non-image areas are not sufficiently durable to permit long printing runs, and are easily scratched. Also, the hydrophilic coatings are not like the traditional hydrophilic grained and anodized surfaces and generally are considered outside the mainstream of conventional printing. One other disadvantage of these plates is that they are negative working, since the portions removed by ablation are the image regions that accept ink. When lasers with a large spot size are used for imaging, the size of the smallest printed dot is as large as the spot size. Consequently, the image quality on printing is not high. For example, a 35 micron laser spot size would print its smallest dot size at 35 microns with a negative working plate. On a 200 lines per inch (lpi) halftone screen, this is equivalent to a 5% to 6% dot.
U.S. Pat. No. 5,493,971 extends the benefit of the traditional grained metal plate to ablative laser imaging and also provides the advantage of a positive working plate. These plates are positive working since the portions not removed by ablation are the image regions that accept ink. This construction includes a grained metal substrate, a hydrophilic protective coating which also serves as an adhesion-promoting primer, and an ablatable oleophilic surface layer. The imaging laser interacts with the ablatable surface layer, causing ablation thereof. When lasers with a large spot size are used for imaging, the size of the smallest printed dot can be very small since the large spot size laser beam can be programmed to remove material around a very small area. Although the smallest hole in a solid printed area is large, this does not seriously affect print quality since very small holes in solids tend to fill in with ink. Consequently, the image quality on printing is high. After imaging which removes at least the surface layer and also at least some of the hydrophilic protective layer, the plate is then cleaned with a suitable solvent, e.g., water, to remove portions of the hydrophilic protective layer still remaining in the laser-exposed areas. Depending on the solubility properties of the residual plug of the partially ablated hydrophilic protective layer in the cleaning solvent, including solubility changes from the damage caused by the laser exposure, the cleaning reveals the hydrophilic protective coating at less than its original thickness, or reveals the hydrophilic metal substrate in the cases where the hydrophilic protective coating is entirely removed by the cleaning solvent. After cleaning, the plate behaves like a conventional positive working grained metal wet lithographic plate on the printing press.
However, adhesion of the remaining oleophilic surface coating to the hydrophilic protective layer has proven a difficult problem to overcome. Loss of adhesion can result if the protective hydrophilic thermal barrier layer in the non-image areas of the plate is damaged or degraded during laser imaging. Too much solvent or solubilizing action by the cleaning solution or the fountain solution on press can erode the walls, eliminating the underlying support provided by the hydrophilic barrier layer around the periphery of the image feature and degrading small image elements. This leads to a major loss of image quality. Small dots and type are often removed during cleaning or early in the print run. Efforts to improve the adhesion of the ablatable surface coating and/or its durability to permit longer printing runs typically leads to a significant increase in the laser energy required to image the plate.
U.S. Pat. No. 5,605,780 describes a lithographic printing plate comprising an anodized aluminum support having thereon an oleophilic image-forming layer comprising an infrared-absorbing agent dispersed in a film-forming cyanoacrylate polymer binder. The hydrophilic protective layer has been eliminated. The '780 patent describes low required laser energy, good ink receptivity, good adhesion to the support, and good wear characteristics. Print runs of more than 8,200 impressions are shown in the examples.
Despite the many efforts directed to the development of a laser imageable positive working wet lithographic printing plate, there still remains a need for plates that require no alkaline or solvent developing solution, that look and perform like a conventional lithographic printing plate on press, that are sensitive to a broad spectrum of laser energy (700 nm to 1150 nm), that provide a high resolution image, and that will be long running at high resolution on press (greater than 100,000 impressions).